This invention is directed to catalysts useful in the conversion of carbon monoxide.
Synthesis gas represents one of the most important feedstocks of industrial chemistry. It is used to synthesize basic chemicals, such as methanol or oxyaldehydes, as well as in production of pure hydrogen.
Synthesis gas produced by steam reforming of hydrocarbons generally does not meet the requirements for further use with respect to CO/H.sub.2 ratio. It is therefore industrial practice to reduce the CO content by conversion with steam.
The conversion of carbon monoxide with steam. EQU CO+H.sub.2 O.revreaction.H.sub.2 +CO.sub.2
is favored by lower temperatures and the reaction is slightly exothermic.
While lower temperatures favor more complete carbon monoxide conversion, higher temperatures allow recovery of the heat of reaction at a sufficient temperature level to generate pressure steam. For maximum efficiency and economy of operation, many plants contain a high temperature reaction unit for bulk carbon monoxide conversion and heat recovery, and a low temperature reaction unit for final carbon monoxide conversion.
Chromium-promoted iron catalysts are normally used in the first stage at temperatures above about 350.degree. C. to reduce the CO content to about 3-4% (cf., for example, D. S. Newsom, Catal. Rev., 21 (1980), 275). As is known from the literature (cf., for example, H. Topsoe and M. Boudart, J. Catal., 31 (1973), 346), the chromium oxide promoter combines two functions. In the first place, it serves to enhance catalytic activity and in the second place acts as a heat stabilizer, i.e., it increases the heat stability of magnetite, the active form of the catalyst, and prevents unduly rapid deactivation under conditions of technical use.
Unfortunately, when chromium is used, especially in hexavalent form, expenditures not to be underestimated must be incurred to guarantee work safety both during production and later handling of the catalyst, and a health hazard cannot be fully ruled out despite considerable effort. In addition, the spent catalyst ultimately poses a hazard to man and the environment and must be disposed of with allowance for the provisions in force for toxic waste.
Particularly, in the production of pure hydrogen, it is necessary to further reduce the CO content of synthesis gas. To this end, the product is fed to a second conversion stage at much lower temperature and thus more favorable equilibrium. Standard catalysts for this conversion stage are based on Cu--Zn oxide. This type of catalyst is indeed devoid of environmentally hazardous components, but is beset with the major drawback of extremely low heat stability so that its use is essentially limited to temperatures below about 250.degree. C. EP B 0 296 734 describes catalysts for conversion of carbon monoxide based on copper oxide and zinc and/or magnesium compounds, optionally with addition of aluminum oxide and other oxides. In preparing the catalysts, the preliminary stages are heated to temperatures of at most 200.degree. C. Under such conditions, spinel formation, which causes thermal stabilization of the active components, does not occur.
The objective of the present invention is to find a catalyst for a CO conversion process that, on the one hand, contains no components that are harmful to man or the environment and, on the other hand, possesses much higher heat stability than available conversion catalysts.